The potent graft versus leukemia effect (GVL) associated with allogeneic bone marrow transplant (BMT) can produce lasting remissions in patients with myeloid leukemia, but the potentially lethal complication of graft versus host disease (GVHD) limits the effectiveness of this treatment. Donor T cells mediate both GVL and GVHD, although the target antigens recognized by these T cells are not known. We hypothesize that GVL would be enhanced and GVHD reduced or eliminated if the target antigens that drove those responses were identified and if T cells with GVL antigen specificity could be isolated and adoptively transferred to leukemia patients. We identified the first human leukemia-associated T cell antigen as PR1, an HLA-A2-restricted nonamer peptide derived from proteinase 3, an aberrantly expressed myeloid-restricted protein in leukemia cells. PR1/HLA-A2 tetramers were used to identify PR1-specific cytotoxic T lymphocytes (CTL) in chronic myeioid leukemia (CML) patients in cytogenetic remission after either interferon or BMT treatment. Using the PR1 peptide as a vaccine, we have shown that tolerance can be reversed and PR1-specific CTL can be elicited in patients with AML and CML. In addition, clinical responses with molecular remission can be achieved in some patients with an immune response to the vaccine. Therefore, PR1 is well established as a leukemia-associated antigen in HLA-A2+ patients with AML and CML. Although Imatinib mesylate induces hematological and cytogenetic remission in the majority of CML patients, it is not clear whether patients are cured. Furthermore, patients with accelerated phase and blast crisis CML seldom achieve lasting remissions and resistance to the drug has been described. Immune-based therapies with low toxicity could be useful in combined treatment strategies to convert patients with refractory disease to molecular remission. In this proposal, we will (1) determine an optimal method to expand PR1-CTL ex vivo that can be used for adoptive immunotherapy, (2) determine whether PR1-CTL can be adoptively transferred to patients that receive allogeneic BMT but who continue to have detectable disease post-BMT to enhance GVL and reduce GVHD, and (3) determine whether the PR1 vaccine can be used to elicit PR1-CTL after autologous BMT for those patients with advanced disease with no suitable donors for allogeneic BMT.